Offshore drilling vessels are widely used for the exploration and exploitation of hydrocarbon reservoirs under the sea floor. One type of drilling vessel structure is a drillship, e.g. as described in WO 2014/108541.
It is generally desirable to provide an offshore drilling vessel that allows for an efficient and flexible operation. It is further generally desirable to provide an offshore drilling vessel that facilitates operation with a high degree of safety. It is further generally desirable to provide drilling vessels that can perform drilling operations at large depths.
The tasks performed by such drilling rigs often include well completion and intervention operations where subsea trees are installed. Examples of such tasks include well completion, flow testing, well stimulation, well workover, diagnostic well work, bullheading operations, plugging wells and/or abandoning wells.
During such operations, a Completion Work-Over Riser (CWOR) system is typically used if a vertical xmas tree is utilized. Such as system comprises a subsea tree that is installed on the seafloor and connected to the well head and a string of completion riser joints spanning between the subsea tree and the drilling vessel. The completion riser typically has an outer diameter of about 8-14″. The CWOR is typically connected to a surface flow tree on the drill floor where hydrocarbons from can be received. The completion riser string is typically suspended from the drilling vessel both by a riser tensioner system that is installed under the drill floor of the vessel and the hoisting system in a shared load configuration. The riser tensioner system typically carries the majority of the load of the completion riser string and maintains suitable tension. The hoisting system of the drilling rig typically carries a minor portion of the load of the completion riser. The riser tensioner assembly typically comprises a number of hydraulic cylinders and/or wires whose one end is connected to the support structure of the vessel and whose other end is connected to a riser tension ring through which the completion riser extends and in which the completion riser can be hung off.
During control and intervention operations, situations may arise where the completion riser string needs to be rapidly disconnected from the subsea tree. To this end, subsea trees typically comprise an emergency disconnect mechanism.
For example, U.S. Pat. No. 8,297,359 discloses a completion work-over riser system where the subsea tree is connected to a lower riser package (LRP) and an emergency disconnect package (EDC) between the CWOR and the tree. The LRP and EDC together implement an emergency disconnect functionality. The emergency disconnect package is installed on top of the lower riser package and, during an emergency disconnect operation, the connection between the emergency disconnect package and the lower riser package is disconnected. Hence, the completion riser string with the emergency disconnect package attached to its bottom end are disconnected from the lower riser package and, thus from the remaining parts of the subsea tree and from the well head.
As an alternative a so-called horizontal xmas tree may be used. In such cases completion and work over operation are typically performed by installing a blow-out-preventer (BOP) and a marine riser between the drilling rig and the xmas tree. The riser tensioners will in this case tension the marine riser whereas the hoisting system supports a high pressure conduit (such as a high pressure casing or high pressure riser) connected to the surface flow tree. The high pressure conduit typically has an outer diameter of about 8-14″. This conduit is connected to a subsea test tree installed inside the assembly typically mainly in the lower BOP stack. In the event of an emergency disconnect, the rams of the BOP will typically be used to seal the well and the lower-marine-riser package of the BOP will be disconnected from the lower stack thereby disconnecting the riser and the high pressure conduit. Subsea test trees typically have a shear-able component which is to aligned with a shearing ram of the lower stack of the BOP. This shearing ram is used to shear the subsea test tree in case of an emergency thus releasing the high pressure conduit from what is typically the main part of the tree below the shear-able component.
In the following the term high pressure (HP) tubular string will be used as a common term for a completion riser as used for vertical xmas trees or a high pressure conduit as used with horizontal xmas trees.
As part of the emergency disconnect procedures tubulars and/or components may be sheared below the point of disconnect of the subsea components, i.e. typically below the EDS in case of a vertical xmas tree and below disconnect point of the LMRP in case of a horizontal tree. In order to avoid that lateral movement of these stumps below the point of disconnect at the time of disconnect (e.g. stumps protruding into the LRP or lower stack when disconnecting the EDP or LMPR, respectively) damages the subsea equipment (e.g. LRP or lower stack of the BOP), it is desirable to lift the disconnected HP tubular string sufficiently so as to allow these stumps to free from the subsea equipment remaining on the seabed. At the same time it is desirable to prevent an uncontrolled upwards recoil of the suddenly disconnected HP tubular string. To this end, the riser tensioner system may be provided with an anti-recoil valve which is configured, in the event of an emergency disconnect, to cause the hydraulic cylinders of the riser tensioner system to pull up the riser tension ring and, thus, the completion riser string or marine riser, by a certain amount in a controlled fashion while preventing an uncontrolled upwards recoil, as this may otherwise damage on-board equipment or structures. For example U.S. Pat. No. 8,157,013 discloses an example of a tensioner system with recoil control.
However, the above anti-recoil arrangement has a number of disadvantages:
Firstly, the above arrangement does not provide sufficient re-coil protection to the parts of the hoisting system, e.g. a top drive, that carry the load of the upper portion of the riser string which extends upwards from the riser tension ring. To this end, an additional riser tension frame may have to be installed above the drill floor and below the top drive or hook of the hoisting system. For example, US 2014/0331908 discloses a tension frame including an anti-recoil valve. However, this solution increases the complexity of the system. It would thus be desirable to provide a less complex system.
Secondly, the riser tensioner system is typically dimensioned such that it can support the weight and outer dimensions of a marine riser string that is used during the drilling stage. As the completion riser string is usually considerably smaller and lighter, the over-dimensioned riser tensioner system may induce undesired stress in the subsea equipment and/or may require adaptation when used with a completion riser string. As the hydraulic cylinders of the riser tensioner system are typically symmetrically arranged to evenly support the load carried by the riser tension ring, it is normally not desirable to disconnect individual cylinders, as this would cause a less symmetric distribution of forces. Thirdly, the compensating tension frame will often have to work in unison with the riser tensioners which requires installation and testing. Fourthly, the controls for the compensating tension frame are typically separate controls from the controls for the riser tensioners which typically are controlled by the driller which in turn may be a source of discoordination. Finally, limited lifting height of the rig may pose a problem for some drilling rigs in application of a compensating tension frame as the frame must be lifted to allow sufficient room to stroke out in case of higher waves or a drift off from position. In some instances the surface flow tree mounted at the end of the HP tubular string also requires added height of the frame which tend to increase as pressure and temperature in the well increases.
It would thus be desirable to provide a system that is more easily scalable, simple and/or safe.